A black, raven-sized raptor with a white band at the base of the tail and chestnut/rust colored upper shoulders best describes a:

The description provided in the question aligns closely with the Harris's Hawk, which is known for its social behavior and distinct physical characteristics. The Harris's Hawk is often identified by its dark plumage, similar in size to a raven, with notable white markings at the base of its tail. The chestnut or rust-colored upper shoulders is a key identifying trait that sets it apart from other raptors. This coloration is prominent and helps in recognizing the species in the field.

In contrast, the Cooper's Hawk is a smaller raptor known primarily for its long tail and distinctive body shape rather than the coloration described. The Red-shouldered Hawk, while having some similar features, typically displays a more pronounced reddish barring across its entire body rather than just focusing on the shoulders. The Broad-winged Hawk, although a raptor, lacks the specific color features noted in the question and has its own unique markings and size considerations that do not match the description as accurately.

Understanding these physical characteristics helps in the accurate identification of various raptor species, making it essential for aspiring falconers and bird watchers.